Published German utility model registration G 9,003,458.9 discloses a main objective of variable focal intercept for surgical microscopes. This main objective essentially comprises two optical units which can be displaced relative to each other. The focal intercept of the main objective on the object side can be varied in dependence upon the relative displacement. However, for a variation of the focal intercept of this kind, there results, inter alia, also a variation of the stereo angle.
With increasing requirements imposed upon the optical power capacity of surgical microscopes, a varying stereo angle, which occurs in dependence upon the adjusted focal intercept of the main objective and therefore a varying physiological viewing impression, is disadvantageous for the surgeon.